Azure provides built-in diagnostics to assist with debugging Node.js applications hosted in Azure App Service Web Apps. In this article, you will learn how to

  • Finding Error Info
  • Useful Logs and Enable stdout/stderr logs
  • Remote Debug
  • Debug info in Response Header

###Finding Error Info: If you are receiving a 500 error on your node.js webapp, Here are few things which you can try to get more info

####Watch error info on web page Include below line of code in iisnode.yml file at webapp root folder D:\home\site\wwwroot.

devErrorsEnabled: true

After including above line, restart your web app and You would start seeing something like below on web browser.

iisnode encountered an error when processing the request.

 HRESULT: 0x6d
 HTTP status: 500
 HTTP subStatus: 1013
 HTTP reason: Internal Server Error

Details on status and substatus codes https://azure.microsoft.com/en-us/documentation/articles/app-service-web-nodejs-best-practices-and-troubleshoot-guide/#iisnode-http-status-and-substatus

####Using Failed request tracing in azure webapps

  • Select your web app in Azure portal.
  • Click on Diagnostic logs in webapp settings and Turn On Failed Request Tracing in Diagnostic Logs Tab. failed request tracing
  • After turning on Failed Request Tracing, Access your error page in browser.
  • This would create new folders(W3SV**) containing failed request logs @ D:\home\LogFiles\ in kudu console(https://<Your_Website_name>.scm.azurewebsites.net/DebugConsole).
  • Failed request logs would provide you more meaningful info about application error. Below is a sample screenshot FREB logs

###Useful Logs :

To Troubleshoot above issue, below logs may help you Uncaught Exception: All uncaught exceptions are by default written to logging-errors.txt file in D:\home\LogFiles\Application folder. You can view them using kudu console(https://Your_Webapp_name.scm.azurewebsites.net/DebugConsole). uncaught exception ####stdout and stderror: stdout: console.log(“log content”) Standard out Log Content would be visible in XXX-stdout-xxx.txt file @ D:\home\LogFiles\Application folder stderror: console.error(“error content”) Standard Error Content would be visible in XXX-stderr-xxx.txt file @ D:\home\LogFiles\Application folder standard out or error

You can turn-on these stdout and stderr using below two ways

####Using iisnode.yml file : Include below line of code in iisnode.yml file at webapp root folder(D:\home\site\wwwroot).

loggingEnabled: true

####Using Azure Portal :

  • Select your web app in Azure portal(https://ms.portal.azure.com/).
  • Click on Diagnostic logs in settings option and
  • Turn On Application logging in Diagnostic Logs Tab. Diagnostics logs

###Remote debug :

Below content explains how to remotely debug your Node.js application deployed on Azure Web Apps using the node-inspector debugger.

  • Enter below line of code in iisnode.yml file at webapp root folder(D:\home\site\wwwroot). 
    debuggingEnabled: true
  • Check if your web.config file has below rule, else Include it. ```
3) Navigate to `http://your_app_name.azurewebsites.net/server.js/debug`.This should bring up the familiar node-inspector interface for your application, which allows you to set breakpoints, inspect code, etc
![remote debug](/content/images/2016/11/node_inspector.png)

For Advanced configuration on using custom debug url, please refer http://tomasz.janczuk.org/2013/07/debug-nodejs-applications-in-windows.html

###Debug info in Response Header:
1. Enter below line of code in iisnode.yml file at webapp root folder(`D:\home\site\wwwroot`).

debugHeaderEnabled: true

After making above change you would see a url in each response header as in below screen shot. It would provide us insights into state of node.js application.
![debug header](/content/images/2016/11/debug_header.JPG)

**Sample Url :**

http://bit.ly/NsU2nd#iisnode_ver=0.2.19&node=node.exe&dns=RD000D3A7037D6&worker_pid=6056&node_pid=2556&worker_mem_ws=9676&worker_mem_pagefile=31928&node_mem_ws=29872&node_mem_pagefile=29372&app_processes=1&process_active_req=1&app_active_req=1&worker_total_req=21&np_retry=0&req_time=221&hresult=0 ``` Please find more details @ http://tomasz.janczuk.org/2012/11/diagnose-nodejs-apps-hosted-in-iis-with.html